To remove a users access on Advisor you must be at Admin level.
Go to the Settings tab along the top of the screen and then to the Users section along the left hand side as you can see in the first screenshot below. You can search for the user you want to disable the access of in the search box on the top right hand side of the page.
Click on the Green Pencil button beside the user to bring up the edit user window you can see in the second screenshot below. In here you have to click on the No box beside the "Allow User Login" line, in the example below the user has had their access removed.
Users cannot be deleted from Advisor as any events that they have created (Reports, Notes, Targets etc) would also be deleted. You can edit the name of the user if you wish to remove their details from the system.